Electrical conductors having extruded insulating coatings thereon are well known. It is also well known to form electrical cables comprised of a number of individual electrical conductors, each of which has an insulating coating on it. Such cables are formed by helically laying or twisting individual conductors to form the final integral cable. Alternatively, cables comprised of parallel laid single conductors are known. For many applications it is desirable to encase the completed cable in a duct or pipe. It is possible that the duct which surrounds the cable may be formed from an extrudable material of the same general type as is used for the insulating coatings of the various individual conductors. It is also desirable to extrude the duct around the cable as the cable is moving or conveyed through the extrusion die which forms the duct.
One particular use to which cables encased in ducts are put is in underground electrical installations. In such installations, a trench is typically dug and the duct having the cable traversing therethrough is placed in the trench. Thereafter, the trench is covered with earth and the cable and duct are buried. Because the nature of various electrical installations may vary from time to time, and further because of problems or difficulties which may be encountered with respect to various conductors of the cable, or with the cable itself, repair or replacement of underground cables may become necessary.
It is obviously expensive and undesirable to reexcavate the trench in which the cable has been laid. Accordingly, it is highly desirable to provide a cable with a surrounding duct which can be moved relative to the duct such that the cable can be removed from the underground installation while leaving the duct buried in place. In this manner, new cable may thereafter be inserted into the duct without the need for expensive reexcavation.
One particular problem which has been recognized with prior art cables which have been placed in ducts is that the cable sticks or adheres to the duct such that after being placed in an underground installation, the cable may not be moved relative to the duct. It is postulated that the cause of the adherence or sticking between the cable and the duct is that during the extrusion of the duct, the insulating coating of the individual conductors making up the cable contacts with and adheres to the duct before the duct has cooled from the temperatures experienced during the extrusion process. As the duct is being extruded, it is in a molten state. After the duct has fully cooled from the temperatures which it experiences during extrusion, it is possible for the insulating coating of the individual conductors to contact the duct without adhering. Before such cooling has taken place, however, contact between the insulating coating of the conductors and the duct tends to cause adherence. Contact between the cable and the duct while the duct is molten may also cause the duct to deform in localized areas. For example, contact between the cable and the duct while the duct is molten may cause depressions or points of weakness in the duct walls which is undesirable.
Various techniques have been proposed or employed for eliminating adherence between electrical cables and the surrounding duct immediately following the extrusion of the duct and while the duct is still vulnerable. In the predominant approach, the extrusion process is simply operated relatively slowly such that the duct is hardened or cooled at a position more closely adjacent to the extrusion die. Thus, when downstream contact between the cable and the duct occurs adherence is avoided. Obviously, however, the operation of a production line at low speed is highly undesirable from an economic standpoint. Accordingly, efforts have been made to eliminate sticking by techniques other than slow speed operation. One such technique has been an attempt to apply talc to the interior of the just-extruded duct. The application of talc, however, tends to be intermittent or subject to removal by scraping. The presence of the talc precludes adherence between the cable insulation and the duct where it is present. However, because of intermittent coating of the talc, sticking still tends to occur between the cable insulation and the duct, thereby precluding relative movement of the cable and the duct which is highly undesirable in underground applications.
In another approach, cooled or refrigerated air or gas has been injected into the interior of the just-extruded duct in an attempt to cool the duct before contact can occur between the cable and the duct's inner wall. It is believed, however, that sticking problems still persist with this approach.
Still another technique which has been employed has been to coat the cable insulation with silicone. However, the primary material utilized for both the cable insulation and the duct is polyethylene. Silicone can react with the polyethylene insulation and with the duct thus causing deleterious effects to both.
Still another approach which the present applicant has employed has been to apply a polyethylene terephthalate barrier tape between the cable and the just-extruded duct in an attempt to preclude adherence between the two. This barrier tape, however, adds to the overall cost of the final product and is not fully satisfactory in precluding sticking or adherence between the conductor insulation and the duct wall. This is especially true at the time at which it is desired to remove the cable from the duct in an underground installation. In such an installation, the polyethylene terephthalate barrier tape tends to buckle, making removal of the cable from the duct particularly difficult.
In addition to the ineffectiveness of the foregoing approaches, all typically require relatively slow speed operation to provide sufficient time for the duct to cool before it comes in contact with the cable present within it. It would be particularly desirable to provide a method and apparatus for precluding adherence between duct and cable which does not require relatively slow operating speeds.
Another troubling problem which arises in the manufacture of electrical cable in a flexible duct in addition to the adherence and deformation problems mentioned above, is the fact that the duct, due to processing conditions, typically deviates from a particularly desirable round, cross-sectional shape. Keeping the duct in a round condition is important, not only for aesthetic purposes, but more importantly because fittings and connections which mate to the duct have a round configuration. If the duct is extruded and, in the course of further processing, is caused to be out-of-round, the duct and the fittings no longer properly mate.
It would be particularly desirable to provide an improved manufacturing process and apparatus which precludes or at least minimizes the instances in which cable is produced in an out-of-round condition. Moreover, it would be desirable to produce a duct which is consistently round in cross-section, to produce the duct at high speeds and to provide a duct which does not stick or adhere to the cables encased within it. It would be further desirable to produce a duct, the walls of which are not easily deformed by the cable during manufacture.
It is an object of the present invention to provide an improved apparatus for the production of extruded duct having a cable extending therethrough which does not suffer from the aforementioned disadvantages.